The invention relates to an apparatus for the continuous, simultaneous application of coatings of constant thickness to both sides of a material web, having two adjustable spreader blades confronting one another at an angle, between which the material web passes for the proportioned feeding of the coatings, at least one spreader blade being clamped on one side in a chuck and engaging under bias and a corresponding biasing force, at its other, free edge parallel to the clamping line, the material web being guided over the free edge of the other spreader blade, doing so at an acute angle to the direction of movement of the material web.
Such an apparatus has been disclosed, for example, by German Pat. No. 1,906,113. In that apparatus the angles of the spreader blades are adjusted with respect to the direction of movement of the material web and the distance between the spreader blades, by means of two clamping devices, for the purpose of obtaining different biases of the blades against one another. A change in the distance between the spreader blades always results in a change of the attitude angle and hence of the configuration of the spreader blades at the spreading area. Furthermore, due to the resilience of the two spreader blades, it is difficult to keep the lines of engagement of the spreader knives at the same level on both sides of the material web.
U.S. Pat. No. 3,187,718 furthermore discloses an apparatus for the constant application of a unilateral coating of constant thickness to a material web, in which a resilient spreader blade clamped on one side in a chuck engages under bias, at its free edge parallel to the clamping line, the material web being carried over a backing roller and carrying coating material applied to it, doing so at an acute angle with respect to the direction of movement of the material web, and is supported in its median portion along a fulcrum line. The chuck at the same time can be pivoted both about the line of engagement of the spreader blade with the backing roller and about a line outside of the clamping line. By the pivoting about the engagement line, the attitude angle of the unstressed spreader blade can be adjusted, and by the pivoting about the line outside of the engagement line, the bias of the spreader knife can be adjusted with a simultaneous alteration of the attitude angle. A change of the bias and hence simultaneously of the attitude angle can be accomplished also by a horizontal, linear displacement of the clamping bar. A fine adjustment of the spreader blade can be accomplished at the support line by varying the pressure in a hose.
In the application of a coating of constant thickness to a material web by means of a resilient spreader blade, the application weight or coating weight, i.e., the thickness of the coating, depends on the one hand on the bias force with which the spreader blade is urged against the backing roller, and on the other hand on the velocity of the material web, the viscosity of the coating material--a paint, for example--and the geometrical shape of the wedge-shaped gap between the spreader blade and the material web, i.e., on the attitude angle of the spreader blade. Of these magnitudes, the bias of the spreader blade as well as the attitude angle of the spreader blade can be varied for the purpose of adjusting the coating thickness. When the bias force and the hydrodynamic pressure are in equilibrium, a specific distance is produced between the free edge of the spreader blade and the support roller, the application weight being determined in this manner.
With the apparatus of U.S. Pat. No. 3,187,718, a variation not only of the attitude angle of the spreader blade but also of the bias of the spreader against the support roller is possible. An adjustment of the bias of the spreader blade at a constant attitude angle, i.e., constant geometrical conditions between the spreader blade and the support roller, however, is neither possible nor intended.